class Solution {
public:
    int takeAttendance(vector<int>& records) {
        int left = 0, right = records.size() - 1;
        while(left < right)
        {
            int mid = left + (right - left) / 2;
            if(records[mid] == mid)
                left = mid + 1;
            else
                right = mid;
        }
        return records[right] == right ? right + 1 : right; // 这里要判断一下，看看是否缺失的是最后一个元素
    }
};